This invention is generally directed at a cap for sewage digesters. This invention is specifically directed at a sewage digester cap assembly which floats on an envelope of gas generated by decomposing sludge, the cap held in place by removable ballast anchors.
Heretofore, it was common to use gas holding covers having solid concrete blocks attached to the lower end of the side skirt of the cover to add extra weight to the cover to increase the pressure of the gas contained within the digester. When the concrete block skirt ballasts were submerged in the sludge, a buoyant force was exerted by the sludge upon the skirt members. Concrete used in ballast members generally has a density of about 150 pounds per cubic foot. A cubic foot of concrete in a submerged condition in sludge has a specific gravity of about 1.4 and an effective weight which is reduced by the weight of a cubic foot of sludge. Thus, in a submerged condition, one cubic foot of a solid concrete ballast exerts a downward force of about 87.6 pounds.
When these ballast members emerge from the sludge, then the effective weight of the concrete is its normal density, i.e. about 150 pounds per cubic foot. Typically, the operating pressure of a digester is that of the skirt ballast when it is raised above the corbels, but still in a submerged condition. If the ballast members are fully emerged from the sludge, the pressure generated is great enough to "blow" the system and the gas dissipates into the atmosphere.
Prior sewage digesters having a tank and a cap are provided having as an internal sealant a water seal between the tank and cap. Digested sewage produces methane gas. The cap contains that gas until such time as it is bled off for secondary use. Additional current models use external concrete ballast to offset the pressure of the gas inside the digester on the cap. The concrete is placed in a box adjoining the perimeter of the cap. Once the cap is in place, the "skirt" adjoining the perimeter of the cap is filled with concrete. Removal of the cap for cleaning, etc., thereafter is difficult. The cap travels a limited distance up and down within the digester, ex. 4 ft. At a predetermined pressure, the travel is limited by the high and low liquid levels into the digester. Travel beyond the high liquid level requires an increase in the pressure to raise the additional weight causing the system to "blow", releasing stored gas into the atmosphere. The increased pressure pulls the ballast out of the liquid so that instead of providing the submerged weight of 87.6 lbs./cu.ft. as ballast, the un-submerged concrete provides 150 lbs./cu. ft. as ballast. Increasing the amount of gas increases the internal pressure to a breaking point.
Numerous sewage digesters have been provided in the prior art that are adapted to include an open tank with a roof having peripheral flanges that travel vertically up and down the tank wall in response to either change in pressure or change in volume within the tank. A roof can be of a limber sheet or a rigid member. A central support, or corbel, inside the tank, supports the roof at a minimum level. Spring biasing means have been used to provide stability to the roof in prior art models to insure its remaining in a properly balanced condition resisting forces exerted on the roof in a direction throwing off balance of the roof. The roof is sealed by the liquid on which the roof floats.
Another digester discloses a floating cover arrangement that is provided with a pontoon construction that defines the peripheral wall. An additional digester has a flexible diaphragm that is vertically movable as a floating roof. An additional digester teaches telescoping or expansion of a movable roof or with counterbalancing plurality of weights connected externally by means of a balancing chain. In many digesters, a liquid seal is formed near the bottom edge of the sidewall trapping gas between liquid in the tank and the cover, buoying the cover upwardly. Some digesters have long side-skirts to maintain the ballast in a submerged condition in the fluctuating level of sludge within the digester.
Typically, sludge digesters operate with fresh sludge continuously entering the digester while sludge which has been decomposed exits the digester, either continuously or periodically. Gas is continually evolving within the digester due to the decomposition of organic matter within the sludge. The rate at which gas evolves is generally dependent upon the amount and type of organic matter in the sludge, the temperature of the sludge, the concentration and type of bacteria in the sludge as well as pH.
While these units may be suitable for the particular purpose to which they address, they would not be as suitable for the purposes of the present invention as hereinafter described.